A wind turbine and its components are designed to withstand and to cope with high wind speeds. The wind turbine might be exposed to a wind speed, which is higher than 25 m/s for example. This wind speed is known by wind turbine manufacturers as a typical “wind speed threshold value”.
Quite high mechanical loads are acting on the wind turbines' components if a given wind speed threshold value is reached and even topped.
It is known to change the pitch angle of the rotating wind turbine blades until a so called “feathered position” of the blades is reached if the measured and averaged wind speed reaches the threshold value. Thus the wind load, which is acting on the blades, is reduced and thus a (fatigue) load is avoided.
It is even known to disconnect the wind turbine from the electrical grid if a so called “high wind situation” with a high wind speed is reached at the site of the wind turbine.
As a drawback electrical energy and thus money is lost for the wind turbine operator if the wind turbine is close to this limiting operational range.
Document EP 0 847 496 B1 describes another solution if the “high wind situation” is reached.
According to the method described the nominal output power of the wind turbine as well as the rotational speed of the rotor are continuously reduced if a certain wind velocity is reached. Thus an overload of the wind turbine and its components are avoided. The continuous reduction of the output power and of the rotational speed of the rotor is done in dependency of the rise in the wind velocity. During the procedure the wind turbine stays connected with the grid. Thus the disconnection of the wind turbine is avoided or is at least delayed. As electrical power is still fed into the grid financial losses are minimized or even avoided.
There is a drawback, which is related to this method as described in EP 0 847 496 A. There always will be situations with a wind speed threshold being reached, while the wind turbine and its components are not endangered by fatigue loads at the same time.
According to the EP 0 847 496 A the wind turbine would reduce the rotational speed and even the electrical output power thus potential produced output power is lost for the operator unnecessarily. Thus in those situation the method is not cost-effective.
Document EP 1497556 B1 discloses another control method, where the power output of the turbine is regulated. The rotor speed is regulated within a predefined wind speed range by varying the rotor blade angle. The output power is reduced in excess of a defined wind-speed-dependent threshold value, while a defined rotor blade limiting angle is used as threshold value.
Other attempts to operate wind turbines in high wind speed situations are described in WO 28121775 A, WO 28121776 A and WO 28121778 A.